Silica pigment and method of preparing same



A. PECHUKAS 2,805,956

SILICA PIGMENT AND METHOD OF PREPARING SAME 2 Sheets-Sheet 1 Sept. 10,1957 Filed May 28, 1952 INVENTOR. ALPHONSE PECIIUKAS ATTORNEY Sept. 10,1957 A. PECHUKAS 2,805,956

SILICA PIGMENT AND METHOD OF PREPARING SAME Filed May 28, 1952 2Sheets-Sheet 2 INVENTOR. ALPFMNSE PECHUKAS hw KW ATTOQNEY United StatesPatent SILICA PIGMENT AND METHOD OF PREPARING SAME Alphonse Pechukas,Pittslield, Mass., assignor to Columbia-Sonthern Chemical Corporation,Allegheny County, Pa., a corporation of Delaware Application May 28,1952, Serial No. 290,536

9 Claims. (Cl. 106-288) This invention reltaes to a novel method ofpreparing a silica pigment and to the novel silica pigment therebyproduced. Prior to the present invention it was known that a silicapigment suitable for reinforcing rubber and for other purposes could beprepared by reaction of an acid or acidic reacting material with a metalsilicate, such as alkali metal silicates and alkaline earth metalsilicates. The preparation of such pigments must be performed underspecial conditions of operation. Typical methods by which such pigmentscan be prepared have been described in the following copendingapplications for United States Letters Patent:

Pigments produced according to the processes disclosed in the aboveidentified applications are in the form of finely divided essentiallyamorphous porous hydrated silica flocs which contain in excess of 80percent and usually above 90 percent by weight of Si02, measured on theanhydrous basis (that is, on a basis excluding free and bound water)bound water in the proportion of one mole thereof per 3 to 9 moles ofSiOz and free water in an amount of about 2 to 10 percent by weight. Thesurface area of this silica is 60 to 200 square meters per gram,preferably in the range of 75 to 175 square meters per gram, and itsaverage ultimate particle size is below 0.1 micron, usually in the rangeof 0.015 to 0.05 micron. In general, the product contains less than 1.75percent, preferably less than 1 percent, of NazO, but may contain up toabout 10 percent by weight of an alkaline earth metal or zinc oraluminum (computed as the oxide thereof). Such metal appears in thepigment as an oxide which possibly is in chemical association with thesilica and usually is present in the proportion of about 10 to 150 molesof SiOz per mole of metal oxide.

An electron photomicrograph of this hydrated silica is shown in Fig. 1.The photograph was prepared from an alcohol-water suspension of thesilica. The large ball appearing in the photomicrograph is a polystyrenelatex particle known to have a diameter of about 2600 angstrom units. Asshown, the silica is in the form of loosely linked aggregates of smallerparticles resembling a bunch of grapes. The average ultimate particlesize of this product is about 0.015-0.05 micron.

Two types of water are present in the pigment herein contemplated. Thesetypes are termed bound water" and free water. The term free water, asused herein, is intended to denote the water which may be removed fromthe silica pigment by heating the pigment at a tern Patented Sept. 10,1957 ice perature of C. for a period of 24 hours in a laboratory oven.The term bound water, as used herein, is intended to mean the amount ofwater which is driven off from a silica pigment by heating the pigmentat ignition temperature, for example, 1000 to 1200 C. until no furtherwater can be removed, minus the amount of free water in the pigment.This bound water appears to be chemically combined with the pigment. Forthis reason, the bound water does not come off readily unless dried attemperatures above 400 C. On the other hand, the free water comes offreadily upon drying at normal or slightly elevated temperatures. Someportion of this water will be picked up on standing in atmospheric airof normal humidification. When bound water is removed, however, only asmall portion thereof is reabsorbed.

According to the present invention, a further novel pigment has beenprepared. This pigment has a reduced water content and when incorporatedin rubber compositions yields products of higher abrasive resistance.This pigment may be prepared by heating the pigments produced accordingto the above described processes to effect a controlled removal of watertherefrom while controlling conditions of heating so as to preventexcessive crystal formation.

For a full understanding of the invention and to make clear the type ofpigment which is subjected to treatment according to my invention toproduce my novel pigment, it is advantageous to discuss in some detailsome of the processes of the above identified applications.

Silica to be treated according to this invention may be prepared by alarge number of methods. A particularly efiective method of preparingthe silica pigment herein contemplated involves reaction of finelydivided alkaline earth metal silicate, such as calcium silicate, havingan average ultimate particle size below 0.1 micron, with an acid havingan anion which forms a water soluble salt with the alkaline earth metal.

In the practice of this process, the acid is reacted with the calciumsilicate in an aqueous medium, and sufiicient acid is added to largelydecompose the calcium silicate and to extract the calcium therefrom, andto prevent establishment of a concentration of calcium in the silicaabove about 6 percent by weight of the silica pigment, computed as CaO.Consequently, suflicient acid is used to reduce the pH below about 7,usually in the range of 3 to 5. During the acidification, the slurry ofcalcium silicate may be agitated in order to promote and facilitatereaction. In order to avoid use of excess acid, acid is added in smallportions until the desired pH has been reached, as indicated, forexample, by suitable indicators, such as methyl orange. in general,additions of large cxcesses of acid beyond a pH of 0, for example, areunnecessary. After the reaction of calcium silicate with acid has beencompleted and the pH of the aqueous slurry has been reduced below about4 or 5, the precipitated silica is recovered.

This silica, of course, is present in an aqueous slurry. Seriousproblems arise in the recovery of the silica from the slurry by virtueof the fact that the silica fails to settle rapidly and also filterswith extreme slowness. What is even more important, the finely dividedpigment thus obtained normally has a surface area well above 200 squaremeters per gram.

It has been found that these difficulties may be avoided by readjustingthe pH of the slurry above 5, usually above 6 and generally in the rangeof 7 to 8.5, rarely in excess of 10. This adjustment is advantageous inorder to facilitate separation of the silica from the aqueous medium. Itis also advantageous since it effects a material reduction n the surfacearea of the pigment produced and thereby insures that this pigment willhave the surface area desired.

Following this, the pigment is recovered by settling and/or filtration.Thereafter, the pigment is dried.

In general, the alkali is added to the precipitated silica before thesilica is completely separated from the mother liquor which containsdissolved calcium chloride or like calcium salt. Thus, the processnormally is conducted by adding acid to a slurry or suspension of thecalcium silicate until the pH thereof is reduced to or below, andthereafter alkali, such as sodium hydroxide or like alkali metalhydroxide, is added to the resultant slurry. As a consequence of this,the silica obtained contains an appreciable amount, usually 1 to 5percent by weight of CaO.

In order to obtain a product which has maximum pigmentary reinforcingcharacteristics when used in rubber compositions, it is necessary to usea special form of calcium silicate or similar alkaline earth metalsilicate. That is, some calcium silicates will not produce the resultsdesired. In general, in order to obtain a proper pigment from calciumsilicate, it is necessary to use a precipitated calcium silicate havinga surface area of 50 to 125 square meters per gram and an averageultimate particle size below 0.1 micron, usually 0.02 to 0.06 micron.

The manner in which the calcium silicate has been prepared has adefinite influence upon the character of the silica which is obtainedtherefrom. Thus, it has been found that tensile tests of rubbercompositions containing silica obtained from calcium silicate preparedby reaction of calcium acetate with sodium silicate were lower thanthose prepared as hereinafter described, either because they cured withextreme slowness or for other reason.

The best silica which has been prepared from calcium silicate has beenobtained when the calcium silicate has been prepared by reacting calciumchloride with alkali metal silicate in aqueous medium containing sodiumchloride or like alkali metal chloride. This sodium chlorideconveniently may be in the calcium chloride solution although it mayalso be in the sodium silicate solution. Thus, it is found mostdesirable to react aqueous sodium silicate with an aqueous calciumchloride solution containing sodium chloride preferably in theproportion of at least 0.1 pound, and usually in the range of 0.2 to 0.5pound, of sodium chloride per pound of calcium chloride. Normally, theNaCl content of the solution should be in excess of 2 to 5 grams perliter. Solutions which contain higher sodium chloride content may beused. However, it is rare that the weight of sodium chloride will exceedthe weight of calcium chloride in the solution or will be present inexcess of 100 grams per liter in either solution. The presence of thesodium chloride materially improves the character of the pigment.

The concentrations of the calcium chloride solution and the alkali metalsilicate solution also have a bearing upon the final product. Thus,using a solution in which the sodium chloride content was 0.3 to 0.4pound per pound of calcium chloride, pigments of inferior quality wereobtained when the calcium chloride concentration was 5 or grams perliter. Best pigments were obtained in such a case when the calciumchloride solution contained at least grams per liter, usually in therange of 50 to 150 grams per liter, and using sodium silicate solutioncontaining in excess of 20 grams of SiOz per liter, usually in the rangeof 50 to 150 grams per liter of SiOz. More concentrated solutions,containing up to about 200 grams per liter of CaClz and of SiOz or evenhigher, may be used although best results have been obtained when theconcentration of the CaClz and SiOa solutions is below 200 grams perliter.

The proportion of calcium chloride solution to sodium silicate normallyis suflicient to react with all or at least most of the sodium silicate.In general, the amount of calcium chloride is in stoichiometric excess.However, small excesses of sodium silicate are not objectionable. Thus,it is possible to use sodium silicate 10 to 25 percent in excess of thecalcium chloride although best results are obtained when the calciumchloride is at least in stoichiometric amount. Excesses of sodiumsilicate as high as 100 percent over stoichiometric usuall giveunsatisfactory products. However, even such amounts may be used if thesodium chloride concentration is suificiently high and the rate ofacidification is held within the proper limits. Thus, the adverseeffects of excess sodium silicate may be counteracted to an appreciabledegree by the presence of sodium chloride in the reaction mixturesubjected to acidification.

The precipitation of calcium or other alkaline earth metal silicate infinely divided state, such as is herein required, may be accomplishedwith best results by mixing a stream of aqueous sodium silicate solutionwith a stream of calcium chloride solution under conditions whichsubject the mixture to a high degree of turbulence and almostinstantaneous mixing. The amount of reactants in the respective streamsis proportioned so as to obtain calcium silicate in the desiredconcentration and to establish an excess of calcium chloride over thestoichiometric quantity required to react with the silicate. Oneeffective way to produce the required turbulence is to introduce twostreams closely together into a central area of a centrifugal pump. inthis case, the agitation of the mixture is effected as the introducedstreams of the reactants are thrown radially outward by the pump rotor.In most cases, it is found desirable to limit the feed of the calciumchloride solution and alkaline metal silicate solutions to the pump toan amount below the capacity of the pump. For example, if the pump iscapable of discharging 100 gallons per minute with unlimited How ofliquid to the pump, the amount of reacting solution supplied to the pumpis held at least 10 percent below, and usually 35 percent or more belowthis amount. This appears to aflord a greater degree of agitation of thereacting solutions and to ensure production of calcium silicate havingthe desired fineness.

To ensure production of the calcium silicate in a highly finely dividedstate, alkaline metal silicate having the composition Na2O(SiO2)I, where.r is a number not less than 2 nor more than 4, is preferably used. Thisresults in the production of a calcium silicate having the compositionCaO(SiO2)$, where x is as defined above. However, other calciumsilicates, wherein at is higher or lower, may be used in certain cases.

The resulting silica is a dry powder which is found to be in anextremely fine state of division and is preponderantly silica. Byanalysis, the dried product normally contains above 75 percent SiOz, theusual range being about 7888 percent SiOa.

On the anhydrous basis, the silica concentration of the product is abovepercent, usually being in excess of about percent. The surface area ofthis product ranges between 75 and square meters per "ram as measured bythe Bi'unauerEmett-Teller method of determining surface area.

The pigment usually contains approximately 10 to 15 percent water. Thefree W." or content normally ranges between 2 to 10 percent, the balancebeing bound water.

The pigment prepared according to this method normally contains anappreciable concentration of calcium. This calcium content usuallyranges between V. to 6 percent, computed as calcium oxide. Because ofthis calcium content, the pH of the pigment is stabilized on th:alkaline side. Other impurities, frequently in the i'LLiiQE mi 0.1 to 2percent, such as iron and aluminum 0 .es, sodium chloride, and carbondioxide, usually are present.

The following are typical analyses of silica samples made from variousruns in which calcium silicate prepared as described above is reactedwith hydrochloric acid as above described:

Percentages in the above table are by weight.

The acid used to effect the neutralization or decomposition of calciumsilicate normally is hydrochloric acid. On the other hand, other acidswhich form water soluble anions with calcium may be used. Such acidsinclude hypochlorous acid, hydrobromic acid, nitric acid, nitrous acid,and acetic acid. The following is an example of this process:

EXAMPLE I Streams of aqueous sodium silicate solution containing 100grams per liter of SiOz as Na2O(SiOz)s.se, and calcium chloride solutioncontaining 100 grams per liter of CaClz and 30 to 40 grams per liter ofsodium chloride were fed directly into the central area of a centrifugalpump at 150 F. The rates of flow were adjusted so that calcium chloridewas approximately 10 percent in excess over the stoichiometric quantityrequired for reaction, and so that the amount of liquid supplied to thepump was about 25 percent below the output capacity of the pump. Inconsequence, the solutions were subjected to turbulent intermixing inthe pump.

The slurry of calcium silicate thus produced was introduced into a tankand sufficient hydrochloric acid solution containing 28 percent byweight of HCl was added, with stirring, to reduce the pH of the slurryto 2. Thereupon, sufiicient sodium hydroxide solution containing 40percent by weight of NaOH was added to raise the pH of the slurry to7.5. The precipitated silica was recovered by decantation andfiltration, and was dried in an oven at a drying temperature of 120-140"C. for 12 hours. The free water content of the product was within therange of 3 to 8 percent by weight of the pigment.

It will be noted that the silica pigments may be prepared from materialsother than calcium silicate. Thus, finely divided precipitated magnesiumsilicate, barium siiicate or strontium silicate, as well as silicates ofzinc and other metals of series 3 to 8, group II of the periodic table,which have the surface area properties roughly approximating those setforth with respect to calcium silicate, may be subjected to treatmentwith water soluble acids according to this invention, in order toextract the metals and produce the herein contemplated pigment. In sucha case, the magnesium or like silicate preferably is produced asdescribed above by reaction of the metal chloride solution containing0.1 pound of NaCl per pound of metal chloride.

The surface area of the resulting silica is determined by the pH of theslurry from which it is recovered. Thus, if sufficient acid is added tothe calcium silicate to reduce the pH to as low as 2, for example, thesilica which is thus obtained has an unusually high surface area. On theother hand, when this slurry is treated with alkali to increase the pHto above 5, the surface area reduces as the pH increases, so that whenthe pH is or above, the surface area has fallen to approximately 135.

The following example illustrates this principle:

EXAMPLE 11 Eight liters of calcium silicate slurry containing 100 gramsper liter of calcium silicate, and prepared according to the methoddescribed in Example I, was placed in a 12-liter flask fitted with astirrer. The slurry was heated to 70 C. while being stirred, and thenhydrochloric acid solution having a strength of 3.5 normal was added ata rate of milliliters per minute for 12 minutes. Fifteen minutes afterall the acid was added, 3.5 normal sodium hydroxide solution was addedto the slurry, with agitation, at a rate of 100 milliliters per minutefor 11 minutes. During the addition of the hydrochloric acid and thesodium hydroxide, 100 milliliters of samples of slurry were withdrawn atthe time intervals indicated in the table below, and placed in 4-ouncesample bottles which were then closed. These samples were allowed tostand for 3 days. Thereafter, the pH of the slurry samples was measuredand the slurries filtered on a 100-milliliter Buchner funnel. The timewas noted when the filter cake lost its shine prior to cracking. Thistime interval was taken as the filtration time.

Following filtration of the slurry, the filter cake was washed withdistilled water until free of chloride ions. The washed pigment sampleswere then dried in an oven at (3., ground in a mortar, and the surfacearea of the samples was measured.

The results obtained are summarized in the following table:

TABLE II ACIdIfiCaIZOH and caustzclzatzon of calcium silicate slurrySurface Filtration Area of Time, Minutes Slurry p11 time (Minproductutcs and (Square Seconds) meters per gram) 9. 14 2 23" 93. 4 8. 98 2'53" 92. 7 8. 93 2 30" 96. 0 8. 83 2' 28" 99. 3 8. 72 2" 28" 105. 5 8.563 4" 113. 7 8. 51 3' 35" 119. 8 8. 4e 2' 41" 123. 2 8.35 2' 48" 124.48.20 2 53" 133.2 8.10 3' 23" 132. 5 7. 8O 2' 54" 130. 5 7. 33 2' 52"135. 5 6. 6S 3 30 165. ll 2. 41 12 35" 506 15 BIINUTES ELAPSED TIBIE 2.30 S O" 507 7. (l0 2 51" 138. 2 7. 57 3' 23" 130. 4 7. 91 3' 31" 123. 78.15 3' 18" 126. 2 8. 29 3' 4" 120. 0 8. 4? 2' 55" 120. 7 8. 58 2 50"110.6 8. T6 2' 52" 105. 2 s. 88 2' 40" 99, l3 8. 97 2' 26" 99. 7 9. 09 221" 116. 7 9.14 2' 5" 138. 2 9. 22 1' 52" 186.3 9. 28 52' 208. 5

In the above tests, it Will be noted that the pH of the slurry and notof the dried pigment was obtained. In general, it is found that when thedried pigment is slurried in water, the pH thereof is somewhat higherthan that of the initial slurry. With slurries having a pH above about8, this difference is only minor. On the other hand, with slurrieshaving a pH below 8, the pH of the dry pigment, when reslurried, usuallyis as much as 1 or 2 pH units above that of the pH of the initialslurry.

The above tests clearly indicate the effect of the pH upon the filteringcharacteristics and also the surface area of the ultimate product. Thus,it is usually desirable to effect the reaction under conditions suchthat the pH of the slurry prior to filtration is in excess of 5 and,preferably, the pH should be so adjusted that the dry pigment has a pHabove 7.

The pigments contemplated within the scope of the present invention maybe prepared by other methods. For example, the calcium silicate preparedas described 7 above, and/ or having the properties described above, maybe reacted with an aqueous solution of ammonium chloride. In such acase, the ammonium chloride reacts with the calcium silicate, liberatingammonia and precipitating silica. This process may be practicedaccording to the methods which have been described in copendingapplication Serial No. 204,493, filed January 4, 1951. As described insuch application, the reaction proceeds according to the followingequation:

The reaction of calcium silicate with ammonium chloride may be effectedby adding the ammonium chloride, usually as an aqueous solution, to anaqueous slurry of the calcium silicate. During this addition, the slurryis generally agitated and, ultimately, the slurry is heated to drive offthe ammonia. Usually, this heating is con tinued until all orsubstantially all of the free ammonia has been driven off. Sulficientammonium chloride is used to ensure decomposition of substantially allof the calcium silicate.

Advantageously, the ammonium chloride solution should contain anappreciable amount of ammonia and sodium chloride. Silica which hasespecially valuable properties has been prepared using such a solution.The amount of ammonia and sodium chloride which may be present iscapable of considerable variation. Solutions containing in excess of 5,and usually ranging from to 50 grams per liter of free ammonia, arefound to be suitable. The sodium chloride content of solutions usedaccording to this invention normally exceeds 25 grams per liter ofsolution, usually being in the range of 50 to 100 grams of NaCl perliter of solution.

After reaction of the calcium silicate with the ammonium chloride iscompleted and the resulting free ammonia has been removed, the resultingslurry is treated to recover the pigment suspended therein. This may bedone eflectively by conventional decantation and/or filtrationoperations. In the course of this operation, water soluble salts, suchas calcium chloride, ammonium chloride, etc. are washed from the pigmentand the resulting product is dried at a suitable temperature.

Some improvement in pigment properties of silica produced by reaction ofcalcium silicate with ammonium chloride is obtained when the resultingsilica is subjected to the reaction of acid after removal of ammonia ises sentially complete. These acids should be capable of forming watersoluble compounds with calcium. Typical acids suitable for use arehydrochloric, nitric, acetic, and like acids. Such treatment removes aportion or all of residual calcium, magnesium, iron, aluminum, and otherimpurities, and thus ensures production of a purer product. Followingthis acid treatment, it frequently will be advantageous to neutralizeexcess acidity and to ensure production of a pigment having a pH above 6or 7. The following examples are illustrative:

EXAMPLE III An aqueous solution of sodium silicate was prepared bydiluting 5.88 liters of sodium silicate containing 298 grams per literof SiOz as sodium silicate having the composition NasO(SiO2)3,as, withsufiicicnt water to produce 20.7 gallons of solution. A further solutionwas made by dissolving 1220 grams of calcium chloride and 320 grams ofsodium chloride in 16.0 gallons of water. Streams of these aqueoussolutions were fed directly into the central area of a centrifugal pump,proportioning the rates of fiow so that calcium chloride remained inexcess over the stoichiometric quantity required for reaction with thesodium silicate at all times. After mixing of the two solutions wascomplete, 475 grams of ammonium chloride was added to the resultingcalcium silicate slurry and the slurry was thereafter boiled for about 4hours, at which time the odor of ammonia was very faint. Thereafter, theslurry was washed and filtered, and was dried SiOz at a temperature ofabout 120 C. A white friable product having the following compositionwas produced:

Percent by weight EXAMPLE IV 47.1 liters of sodium silicate solutioncontaining 298 grams per liter of SiOz as N320.(Si02)3.36 was diluted to145 gallons. 87.5 gallons of an aqueous solution containing 10,650 gramsof calcium chloride and 2,800 grams of sodium chloride was made up.These solutions were mixed with vigorous agitation as in Example III.The slurry precipitate was washed to remove dissolved chlorides, and anaqueous slurry containing 42.7 grams of calcium silicate per liter ofslurry was obtained. Fifty gallons of this calcium silicate slurry wasmixed with 23.19 liters of aqueous ammonium chloride solution containing160 grams per liter of NH4C1 and about 20 grams per liter of freeammonia, together with about grams per liter of NaCl.

The resulting mixture was heated to boiling until no further ammonia wasgiven off. Thereafter, the precipitate was filtered, washed, dried, andpulverized. The resulting product is preponderantly SiOz.

EXAMPLE V Calcium silicate slurry was prepared according to the processgenerally described in Example I, using an aqueous solution of sodiumsilicate containing grams per liter of SiOz as Na2O(SiOz)s.s6, andcalcium chloride solution containing 100 grams per liter of CaClz and 30to 40 grams per liter of NaCl at a temperature of 150 F. The resultingslurry of calcium silicate contained 0.35 gram equivalents per liter ofalkalinity as determined by titration with HCl to methyl orange endpoint.

One hundred gallons of this calcium silicate slurry was placed in a tankand mixed with 20 gallons of ammonium chloride solution containing 1.95grams equivalents of NIICi per liter of solution as well as 15 grams perliter of free ammonia and 70 grams per liter of NaCl.

The resulting slurry was heated by passing the slurry in countercurrentcontact with steam in a 6-inch glasslined steel column packed to a depthof 18 feet with /2 inch Berl saddles. in this operation, the slurry wasfed to the top of the column at a rate of 10 gallons per hour, andsubstantially saturated steam fed to the bottom of the column at a rateof 50 pounds per hour, ammonia being withdrawn from the top of thecolumn. The resulting slurry had a pH of 7.6.

After filtration and drying at a temperature of about 100 to C., theproduct had the following composition:

Percent by weight Chloride 0.47 Free H2O 10.45 Ignition loss 12.49 CaO1.85 Balance pH 8.2

This product is an effective rubber reinforcing pigment.

EXAMPLE VI Twelve gallons of silica slurry prepared as described inExample V, after the steam treatment, was mixed with 200 cubiccentimeters of an aqueous solution of hydrochloric acid containing 32percent by weight of HCL asoaosa 9 The resulting mixture was allowed todigest at a temperature of 30 C. for 16 hours. Thereafter, the slurrywas filtered and the resulting silica dried. This product, whenincorporated in rubber, according to standard methods, yielded resultswhich were superior to those obtained using silica prepared according toExample V.

It will also be understood that hydrated silica pigments may be preparedby other processes. For example, silica can be precipitated by reactionof ammonium chloride with sodium silicate under specific conditionswhich are controlled and adjusted so as to produce a pigment of thecharacter herein contemplated. Moreover, sodium silicate may be reactedwith acids, notably carbonic acid, in order to produce a pigment of thetype herein contemplated.

Hydrated silica pigments to be treated according to this invention maybe prepared by direct precipitation of silica from sodium silicate. Insuch a case, they must be prepared carefully in order to avoid gelformation. The precipitation must be effected by adding an acid or acidanhydride, such as carbon dioxide, to an aqueous solution of alkalimetal silicate under carefully correlated conditions. Thus, the rate ofacid introduction must be adjusted in accordance with the SiOsconcentration, the NaCl concentration, and the temperature of the alkalimetal silicate solution. Generally, the silicate solution should contain10 to 100 grams per liter of SiOz as Na2O(SiO2), where x is 2 to 4, andthe temperature should be to 100 C. The rate of acid addition to anysuch solution at a given temperature will depend upon the NaCl contentthereof.

Inorganic acids may be used for this purpose. Such acids include theacid anhydride, carbon dioxide, or sulphur dioxide as well ashydrochloric acid, hypochlorous acid, nitric acid, sulphuric acid,sulphurous acid, phosphoric and phosphorous acids, and acid salts, suchas sodium bicarbonate and the like.

Moreover, certain so-called neutral salts, notably ammonium chloride,may be used.

The following examples are illustrative:

EXAMPLE VII A 30-gallon, open-top barrel, provided with an agitatorconsisting of a vertical shaft driven by a Mr H. P. motor and havingthree 3-inch propellers, was charged with 48 liters of an aqueoussolution of sodium silicate, Na(SiO2)3.3s, containing 20.3 grams perliter of NazO, about 68 grams per liter of SiOz, and 10 grams per literof sodium chloride. Carbon dioxide gas, diluted with air to such anextent that the diluted gas had a CO: concentration of about 10 percentby volume, was introduced into the drum through a stainless steel tubewith the discharge end of the tube being located below the bottom of theagitator. The rate of introduction of gas was adjusted so that just thetheoretical amount of carbon dioxide was introduced into the solution in24 hours, required to produce sodium carbonate. This carbonation ratewas held substantially constant over the carbonation period. Thetemperature was maintained at 35 C. during carbonation and the mixturecontinuously agitated.

After the theoretical amount of carbon dioxide had been introduced, themixture was heated by direct introduction of steam from a l40-poundsteam line to maintain the temperature of the slurry at boilingtemperature for a period of about 2 hours. The heated slurry was thenfiltered and the dewatered silica dried in an oven at a temperature of108 C., after which it was micropulverized.

The surface area of the resulting finely divided silica was determinedby the standard low temperature, nitrogen adsorption method proposed byBrunauer, Emmett, and Teller, and was found to be 149 square meters pergram.

In the preparation of the pigment under the above conditions, thepresence of the soluble salt is important. Silica formed under the sameconditions but with smaller amounts of sodium chloride was inferior as arubber pigment, and when the sodium chloride was omitted, theprecipitated silica gelled and was discarded.

The term theoretical amount is used to designate the calculated amountof carbon dioxide required to convert the sodium of the sodium silicateinto sodium carbonate.

EXAMPLE VIII The procedure of Example VII was followed except that thefiltered silica pigment was Washed with hydro chloric acid beforedrying, and then washed free of chloride ions with water. The acidtreated pigment was dried and micro-pulverized as in Example VII. Themeasured surface area (B. E. T. method) of the resulting pigment was 461square meters per gram. The bound sodium present in the sample was 0.02percent.

EXAMPLE IX A -liter autoclave kettle provided with a heating and coolingcoil, an agitator, and a metal thermometer, was charged with 48 litersof a solution containing 20 grams per liter of sodium chloride and aquantity of sodium silicate suificient to cause the solution to contain20.3 grams per liter of Na2O and about 68 grams per liter of SiOz.Essentially pure carbon dioxide was introduced through the bottom of thekettle under the liquid level of the solution at a point about 1 inchbelow the center of the agitator. The temperature was maintained at 25C. during carbonation.

The carbon dioxide was fed to the solution at such a rate as to deliverthe theoretical amount of carbon dioxide thereto in 4 hours andcarbonation was continued at this rate for an additional hour, thusproviding a 25 percent excess of CO2 over that theoretically required toproduce the carbonate.

After 5 hours of carbonation, a sample of the slurry (designated sampleA in the table below) was taken out, the pigment filtered, washed twicewith water, reslurried, and the pH of the slurry adjusted to 7.3 withhydrochloric acid. Thereafter, the pigment was washed until the filtratewas substantially chloride-free.

The slurry remaining in the kettle was boiled for 1 hour and two samplesof the boiled slurry removed from the kettle. One of these samples(designated sample B in the table below) was washed with water alonewhile the other (sample C) was rc-slurried and the pH of the slurryadjusted to 7.2 with hydrochloric acid. The acidified pigment was thenwashed substantially chloride-free with water.

The slurry remaining in the kettle was maintained under a carbon dioxideatmosphere with agitation for an additional 2 hours. The carbon dioxidepressure was maintained from about 2 to 5 pounds per square inch gauge.This treatment of the slurry with carbon dioxide under pressure reducedthe pH of the slurry somewhat. A sample of the thus treated slurry(sample D) was recovered by filtration and washed with tap water.

All of samples A to D were dried at C. in a forced draft, laboratoryoven, and were micro-pulverized. Their surface areas were as follows:

Surface Area (Square meters per gram) EXAMPLE X A 90-liter autoclavekettle provided with a heating and cooling coil, an agitator, and ametal thermometer, was charged with 12,850 grams of sodium silicatesolution containing 976 grams of NazO and 3115 grams of SiOz. Thesolution was diluted to 48 liters total volume, and the temperatureraised to 95 C. The solution was carbonated with 100 percent CO2 and acarbonation rate was used such as to introduce the theoretical amount ofCO2 in about 30 minutes. Carbonation was continued at this rate forabout 1 hour, at the end of which time the pH of the slurry was 9.85.

The resulting slurry was filtered and washed twice with hot tap water.The filter cake was re-slurried and adjusted to a pH of 6.75 by adding400 cubic centimeters of 3.5 N HCl thereto. The acidified slurry wasthen filtered and the filter cake washed nearly chloride-free with hottap water, after which the precipitate was dried at 105 C. in a forceddraft laboratory oven, then micropulverizcd. air-conditioned, analyzed,and compounded in rubber as previously described. The finished pigmenthad a pH of 8.2 and contained 0.61 percent sodium. Its B. E. T. surfacearea was 148 square meters per gram.

The present invention is directed to a novel method of improving thepigments described above and to the novel composition produced thereby.Rubber compositions reinforced with these pigments have definitelysuperior abrasion resistance and higher modulus characteristics thanrubber compositions containing silica of the type described above.

This improved pigment contains silica and bound water in relativeamounts corresponding to the formula:

Where x is a number (including both Whole and fractional numbers)between 14 and 85. This pigment may be prepared by heating or calciningany of the above described hydrated precipitated silica pigments at atemperature above about 350 C. but below that at which substantialcrystalline silica is formed (usually not above 800 to 900 C. andfrequently much lower) and interrupting the heating before appreciable(more than about 2 to 5 percent) of the silica is converted tocrystalline silica, as determined by X-ray diffraction.

The permissible temperature range and time of heating are affected byimpurities in the pigment treated which, in turn are determined by theprocess of precipitating the pigment. For example, silica prepared byprecipitation from alkali metal silicate solution, as described in theaforesaid application of Thornhill, becomes excessively crystalline whencalcined at a temperature of 800 C. in a muffle furnace for 16 hours.Hence, calcining such pigment under such condition should not exceed 700C. and usually should be in the range of 375 to 700 C.

On the other hand, higher temperatures may be applied without excessivecrystal formation when the silica undergoing calcination has beenprepared by reaction of calcium silicate or like silicate of an alkalineearth metal, etc, according to the methods described in the aforesaidapplications of Allen. With such pigments, the temperature of heating at16 hours calcination time in a mutlle furnace may be as high as 850 C.best results being obtained at 550 to 850 C.

It will be understood that these temperatures apply when the product isheated in bulk in a mufile or hearth furnace. With other methods ofheating, such as spray drying, heating in fluidized beds or inductiveheating with radio frequency or other high frequency electric current,

12 other temperatures can be applicable. The effect of the heating is toreduce the amount of bound water which apparently is chemically combinedwith the silica. On the other hand, the removal of all of the boundwater is undesirable for several reasons.

In the first place, an over-calcined product contains excessivecrystalline silica. This is very objectionable from a health standpoint.Moreover, the abrasion resistance of rubber containing the calcinedsilica herein contemplated is highest when the bound water contentthereof is in the range of one mole of water to 14-85 moles of SiOz.Accordingly, a careful control of the contemplated dehydration isessential.

The exact temperature at which the dehydration is conducted depends uponthe type of heating equipment used and the time of drying. Thus, thepermissible temperature is higher when spray driers, as distinguishedfrom tray driers or rotary calciners, are used.

The drying may be conducted in one or more stages. That is, the pigmentmay be dried at 100 to 200 C. to produce a dry powder, as described inthe above examples, and then calcined according to this invention.Alternatively, the drying may be conducted in a single operation whereinwet filter cake is introduced into a drier and heated to the desiredtemperature.

The resulting pigment has the composition H2O(Sl02).17, as explainedabove. It is a finely divided fluffy product having a surface area of to200 (preferably to 150) square meters per gram and, after standing,normally will pick up a small amount of free water.

The amount of free water which the product contains depends upon thetemperature to which the product has been calcined. Where the silica hasbeen calcined above about 800 C., it displays little tendency to pick upfree water. In fact, silica produced by direct acidification of sodiumsilicate tends to lose its ability to pick up much free water whencalcined above 600-700 C.

The following examples are illustrative of the calcination ordehydration herein contemplated:

EXAMPLE XI About 7.5 pounds of finely divided silica prepared as inExample X is placed in a nickel rectangular-shaped pan which is 11.5inches by 22 inches by 6% inches deep. The pan is placed in a HoskinsElectric Furnace, 220-vo1t, which has a heating chamber 13 inches by 33inches by 8 inches high. This muffie furnace is fitted with an automatictemperature control and calibrated in degrees Fahrenheit. One round ventport 1 inch in diameter is located in the rear of the furnace, and asimilar port is located in the center of the furnace door. The muffle,which is fitted with a thermocouple, is heated to the desired calciningtemperature after the introduction of the sample. The temperature of themufile furnace increases about F. per 10 minutes up to the calciningtemperature. The time of calcining is approximately 16 hours.

At the end of the calcining period, the heating is discontinued. If thecalcining temperature is 600 C. or lower, the sample is removed from thefurnace immediately. If the calcining temperature is greater than 600C., the sample is usually allowed to cool somewhat in the furnace withthe furnace door open to avoid discomfort during handling.

Immediately after removal from the furnace, the sample is covered with anickel lid which fits the nickel calcining pan loosely. The sample isallowed to cool to approximately 100 C., and then usually placed intinned onegalion friction top cans. It is thus stored in the absence ofatmospheric moisture until the time of compounding.

The following table tabulates the results obtained using two samples ofsuch silica, one containing 1.2 percent by 13 weight of sodium, theother having been treated with acid to reduce the sodium content to 0.5percent by weight:

14 ing the composition (Na2O)(SiO)z)z.ss and containing the amount ofsodium silicate equivalent to 20 grams TABLE III Analysis of sample whendried Starting for 24 hours at 105 0. Surface Sample, Area MoistureCrystal- Sample Temperature (Square Regain l llnity, otCaicinetion S10:Bound Mole meters (24 hours) percent l" 0.) (Percent Water Ratio, pergram) byweigbt) (Percent S10;

by weight) to H2O 93.0 3. 55 7.8 None 105 92. 7 3. 86 7. 2 126 4. 2 None200 93. 43 2. 81 9. 5 125 5. 4 None 300 93. 55 2. 97 9. 7 126 5.0 None400 94. 6 1. Q2 15 123 4. 1 None Silica No. 1 contains 1.2 per- 500 95.01.47 21) 119 3.2 None cont Na 600 95.1 1. 28 23 8S 1. 5 None 600 95.3 1.IS 25 91 0.7 None 700 95. 27 1. 21 24 52 1.6 2.4 700 95. 2 O. 90 32 430.7 None 800 96.1 0. 34 83 28 0.1 100 800 96. 19 0. 27 103 6 0. 3 None900 None "166' 'z i 1E """iis' "513? gone 4. 1 one ifl 2 600 95.75 1.1525 101 1. 91 None 1 a 700 95.7 1. 20 24 16 0.1 s 800 95. 6 l. 29 22 180. 1 900 96. 8 3. 1.0 29 2 0. 1 100 1 Moisture regain done at 50%relative humidity and 73.5 F., 24-hour regain reported.

Rubber compositions containing samples of the product calcined at 400600C. have consistently shown better abrasion resistance in test thansimilar compositions containing the uncalcined product.

EXAMPLE XII Silica produced according to Example I and dried in acommercial dried at l-130 C. was calcined as described in Example XII,with the following results:

TABLE IV per liter of NazO, and also containing 5.78 grams per liter ofsodium chloride, was placed in a 4000-gal1on tank equipped with a 25 H.P. turbo agitator. While the temperature of the silicate solution was25C., 500 gallons of the ammonium chloride solution was addedcontinuously during agitation over a period of 4 hours.

The resulting product was heated in a still to drive off ammonia and,after the ammonia was driven off,

Analysis of sample when (tied for 21 hours at 105 0. Surface Area(Square meters per gram) Starting Sample, Temperature oi Calcination C.)Mole S102 (Percent by weight) (Percent by weight) Moisture Regain 1 (21hours) Crystallinity, percent None None None None None None None None None None None None None None 72 100 1 Moisture regain measured byconditioning sam and measuring moisture gained alter 24 hours.

Ammonium chloride solution containing about 44.8 grams per liter oflived NHs as Ni-ltCl, 73.9 grams per liter of NaCl, and 16.2 grams perliter of free NHs, was used as ammonium chloride solution in thisexperiment. 2075 gallons of a sodium silicate solution havie at relativehumidity and 73.5 F. his water is essentially all "free" water.

the product was conditioned by holding the resulting slurry above C. for/2 hour. Thereafter, the slurry was filtered and the filter cake driedat -125" C.

A quantity of the resulting dried product was calcined at 600 C. as inExample XI. A rubber composition containing this calcined silica wasfound to have an abrasion loss of 1.8 cubic centimeters per 1600resolutions, whereas the same rubber composition, when compounded withthe uncalcined silica in the same manner, had an abrasion loss of 2.3cubic centimeters per 1600 revolutions, using a standard method fortesting abrasion.

The above described silica compositions are useful reinforcing pigmentsin various rubber compositions including natural rubber and syntheticrubber compositions including butadiene-l,3-styrene copolymers,butadiene- 1,3-acrylonitrile copolymers, butadiene-isobutylenecopolymers (butyl rubber) and like synthetic elastomers which arederived from polymerization of butadiene-1,3 2-chlorobutadiene,isoprene, ethylene or the like alone or with other polymerizablematerials including styrene, methyl methacrylate, methyl chloroacrylateacrylonitrile, vinyl chloride and there equivalents.

Approximately 5 to 100 parts by weight of silica is used per 100 partsby weight of rubber. Best results are obtained when 40 to 80 parts byweight of silica is used per 100 parts of rubber. The rubber compositionusually contains other conventional components such as accelerators andmodifying agents. The compositions are prepared and cured according tomethods well known in the art.

The following examples are illustrative:

EXAINIPLE XIV Silica prepared as in Example I and silica calcined at 600C. as in Example XII were compounded into rubber according to thefollowing formula:

Parts by weight The above compound was mixed at 315 F. and standardabrasion test specimens were prepared according to standard methods andabrasion resistance of the product was measured. The abrasion resistancefor rubber compounded with uncalcined silica was 6.0 cubic centimetersper 1600 revolutions. In contrast the abrasion resistance of rubberreinforced with the calcined silica was only 3.3 cubic centimeters per1600 revolutions.

Although the present invention has been described with particularreference to the specific details of certain embodiments thereof, it isnot intended that such details should be regarded as limitations uponthe scope of the invention except insofar as included in theaccompanying claims.

What is claimed:

1. Finely divided, precipitated, hydrated, amorphous silica having anaverage ultimate particle size of 0.015 to 0.05 micron and containingSiOz and bound water in the proportion corresponding to the formula:

where .r is a number between and 50, said silica having a surface areaof 50 to 200 square meters per gram and the crystalline silica contentthereof being less than 5 percent by weight, the total SiOz contentthereof being in excess of 90 percent by weight, said silica containingcalcium in the proportion of one mole of calcium oxide per 10 to 150moles of SiO-z and having an average ultimate particle size of 0.015 to0.05 being substantially to claim 6.

3. Finely divided, precipitated, hydrated, amorphous silica having anaverage ultimate particle size of 0.015

micron, said silica identical to that produced according where x is anumber between 15 and 50, said silica having a surface area of 50 to 200square meters per gram and the crystalline silica content thereof beingless than 5 percent by weight, the total SiOz content thereof being inexcess of percent by weight, said silica containing an oxide of a metalof series 3 to 8, group II of the periodic table, in the proportion ofone mole of said metal per 10 to 150 moles of SiOz, said silica beingsubstantially identical to that produced according to claim 5.

4. A method of preparing a silica pigment which comprises heating finelydivided, precipitated, hydrated, amorphous silica containing SiO2 andbound water in the proportion of 3 to 9 moles of SiOz per mole of boundwater and having a surface area of 50 to 200 square meters per gram, thetotal $102 thereof on the anhydrous basis being in excess of 80 percentby weight, said hydrated silica having an average ultimate particle sizeof 0.015 to 0.05 micron, at a temperature from about 375 C. to 900 C.until the amount of bound water and silica therein corresponds to theformula:

H2O. (Sl02)x where x is a number between 14 and 85, and interrupting theheating before the content of crystalline silica thereof exceeds 5percent by weight.

5. A method of preparing a silica pigment which comprises heating finelydivided, precipitated, hydrated, amorphous silica containing SiOz andbound water in the pro portion of 3 to 9 moles of SiOz per mole of boundwater and having a surface area of 50 to 200 square meters per gram, thetotal SiOz content thereof on the anhydrous basis being in excess of 80percent by weight, said hydrated silica having an average ultimateparticle size of 0.015 to 0.05 micron and containing less than 1.75percent by weight of NazO, at a temperature from about 375 C. to 900 C.,until the amount of bound water and silica therein corresponds to theformula:

where x is a number between 14 and 85, and interrupting the heatingbefore the content of crystalline silica thereof exceeds 5 percent byweight.

6. A method of preparing a silica pigment which comprises heating finelydivided, precipitated, hydrated, amorphous silica containing SiOz andbound water in the proportion of 3 to 9 moles of SiOz per mole of boundwater and having a surface area of 50 to 200 square meters per gram, thetotal SiOz content thereof on the anhydrous basis being in excess of 80percent by weight, said hydrated silica having an average ultimateparticle size of 0.015 to 0.05 micron and containing CaO in theproportion of 1 mole of CaO per 10 to 150 moles of SiOz and containingless than 2 percent by weight of NazO, at a temperature above 500 C. butnot in excess of 900 C., until the amount of bound water and silicatherein corresponds to the formula:

where x is a number between 14 and 85, and interrupting the heatingbefore the content of crystalline silica thereof exceeds 5 percent byweight.

7. The process of claim 4 wherein the temperature of heating is 500 to900 C.

8. The process according to claim 4 wherein the hy drated silicasubjected to heating is substantially identical to that produced bymixing flowing streams of aqueous sodium silicate solution containinggrams of SiOa as sodium silicate (Na2O(SiO2)3.3a) and of calcium chloride solution containing 100 grams of calcium chloride and 30 to 40grams of sodium chloride per liter of solu tion together in the centralarea of a centrifugal pump at F. while adjusting the rate of flow sothat the calcium chloride is approximately 10 percent in excess of thestoichiometric quantity thereof required for reaction and therebyforming finely divided, precipitated calcium silicate having an averageultimate particle size below 0.1 micron, reacting the resulting calciumsilicate with sufficient hydrochloric acid solution to reduce the pH ofthe resulting slurry to 2, thereupon adding to the slurry enough sodiumhydroxide to raise the pH of the slurry to 7.5, and recovering theprecipitated pigment.

9. A method of preparing a silica pigment which com prises heatingfinely divided, precipitated, hydrated, amorphous silica containing SiOzand bound water in the proportion of 3 to 9 moles of SiOz per mole ofbound water, and a total SiOz content on the anhydrous basis in excessof 80 percent by weight and having a surface area of 50 to 200 squaremeters per gram, an average ultimate particle size of 0.015 to 0.05micron, and containing an oxide of a metal of series 3 to 8, group II ofthe periodic table, in the proportion of 1 mole of said metal per 10 to150 moles of SiOz, at a temperature of from about 375 C. to but not inexcess of 900 C., until the amount of bound water and silica thereincorresponds to the formula H2O.(Si02), where x is a number between 14and 85, and interrupting the heating before the content of crystallinesilica thereof exceeds 5 percent by weight.

References Cited in the file of this patent UNtTED STATES PATENTS1,259,806 Tone Mar. 19, 1918 1,270,093 Arsem June 18, 1918 1,347,191Thompson ct a1. July 20, 1920 1,819,356 Church Aug. 18, 1931 1,842,394Endres Jan. 26, 1932 1,903,187 McClenahn Mar. 28, 1933 2,211,510 MeinckeAug. 13, 1940 1,237,374 Smith Apr. 8, 1941 2,496,736 Maloney Feb. 7,1950 2,649,388 Wills et a1. Aug. 18, 1953 2,679,463 Alexander et a1. May25, 1954 2,686,731 Wainer Aug. 17, 1954 FOREIGN PATENTS 675,341 GreatBritain July 9, 1952 OTHER REFERENCES Mellor: Comprehensive Treatise onInorganic and Theoretical Chemistry, Longrnans Green & Co., New York,1925, vol. 6, page 359.

Shapiro et al.: Thermal Aging of Precipitated Silica (Silica Gel);contribution from the School of Chemistry of the University ofMinnesota; February 1950.

1. FINELY DIVIDED, PRECIPITATED, HYDRATED, AMORPHOUS SILICA HAVING ANAVERAGE ULTIMATE PARTICLE SIZE OF 0.015 TO 0.05 MICRON AND CONTAININGSIO2 AND BOUND WATER IN THE PROPORTION CORRESPONDING TO THE FORMULA: